Previously, papermakers' belts for use in the forming section of a papermaking machine have been made with metal warps since such strands are resistant to stretch. However, they have not continued to be widely used because they are subject to damage as a result of crinkling or creasing during shipment and installation. Also, such belts have low flexural fatigue resistance as compared to synthetic belts.
In order to attempt to overcome the forementioned problems synthetic papermakers' forming fabrics were introduced into the papermaking field. Such fabrics have incorporated relatively low modulus load bearing yarns made of nylon, polyester, rayon and the like. By low modulus yarns or material it is meant yarns or material having a modulus of elasticity of less than 150 grams per denier measured as defined herein. However, problems have occurred using such fabrics in some papermaking applications where such fabrics are subjected to high longitudinal tension causing high drag and with resulting stretch in the longitudinal direction. For example, since the forming fabrics transmit energy from the drive roll to the drainage elements and other rotating and stationary parts of the forming section of the papermaking machine, it is essential that such fabrics possess a high degree of resistance to stretch in order to prevent power losses resulting from the belt slipping or creeping on the drive roll. Further, excessive stretch may result in the fabric "running" off the rolls of the papermaking machine causing a consequent loss of papermaking machine use time. Even relatively minor stretching of papermaking fabrics results in various operational problems such as increased wear. Consequently, it has been a long-sought objective in the papermaking art to produce belts which are resistant to stretching.
Unexpectedly, it has been found that by using ultra high modulus yarns as the load bearing yarns of a papermakers' fabric it is possible to obtain advantages not before possible. By load bearing is meant those yarns which extend in the machine or longitudinal direction which is the direction of travel of the belt. In the context of the present invention by ultra high modulus yarns or material it is meant yarns or material comprised of an organic polymer and having a modulus of elasticity greater than 150 grams per denier as measured at 1% elongation (using zero % elongation as a reference point) by dividing the force in grams per denier at 1% elongation by 0.01. Papermakers' belts incorporating such ultra high modulus yarns as load bearing yarns are substantially stretch resistant in the machine direction even when subjected to high tension applications, and may have an extensibility in the range of less than 0.40% and may be as low as 0.18% or less, making it possible to produce a belt having better wear characteristics and an increase in the overall efficiency of the papermaking operation. In addition, the use of ultra high modulus yarns as load bearing yarns allows for reduction relative to prior art papermakers' belts of the diameter of the load bearing yarns, or number of machine direction or load bearing yarns per inch of width, needed to insure adequate dimensional stability and stretch resistance. Such a reduction in the number or diameter of load bearing yarns per inch of width permits an increase in the number of non-load bearing yarns per inch of length. By non-load bearing yarns is meant those yarns which extend in a direction transverse to the load bearing or machine direction yarns. By increasing the number of non-load bearing yarns per inch and decreasing the number of load bearing yarns per inch it is possible to produce a belt the interstices of which are more square than prior art synthetic papermakers belts since a belt may be produced having a more equal number of warp and weft yarns per square unit of the belt. Belts which are more square have better pulp fiber retention as well as desirable wear characteristics and transverse or cross machine direction stability. The desirable wear characteristics mentioned result from the use of additional wear, non-load bearing transverse yarns permitted by a reduction in the diameter or number of load bearing machine direction yarns needed which can be achieved when ultra high modulus load bearing yarns are used. This also allows for an increase in the cross machine direction stability because the number of cross machine direction or non-load bearing yarns in the belt can be increased. Other desirable operational characteristics which are enhanced by the use of ultra high modulus yarns are a more uniform support of the paper sheet since the fabric is more square and therefore there is less marking of the paper sheet. If desired smaller diameter ultra high modulus yarns can be used so as to produce a belt having smaller interstices and hence even greater pulp fine retention and sheet support as well as less sheet marking than structures using ultra high modulus yarn diameters equivalent to those normally used in papermakers' belts.
Further, to the extent that it is possible to reduce the number of load bearing yarns per inch of width loom productivity may be increased effecting cost savings in the production of belts incorporating such yarns.
Thus it is an object of this invention to provide a papermakers' belt comprising ultra high modulus load bearing yarns.
It is a further object of this invention to provide a papermakers belt having stretch resistance.
It is a still further object of this invention to provide a papermakers' belt which is more square and in which the number and diameter of load bearing yarns can be reduced and the number of wear bearing yarns increased.
Another object of the present invention is to provide a papermakers' belt having improved wear characteristics and better retention of pulp fibers.
A still other object of the invention is to provide a means of increasing loom productivity.
A further object of the present invention is to provide a belt having desirable dimensional stability.
A still further object of the present invention is to provide a fabric with reduced tendency to mark the sheet.